In a conventional image forming apparatus such as a copying machine or printer, the laser scanning optical device for forming an electrostatic latent image on a photoreceptor has often used a long-sized optical element as an optical element for providing the fθ characteristic (a characteristic for ensuring that an optical beam deflected by a polygon mirror or the like is scanned on a surface to be scanned at a equidistant speed), for example. In such an optical element, a prescribed warping is kept with a high accuracy particularly in the scanning direction so that the optical path of the laser beam is adjusted; for example, the main scanning speed is adjusted when the laser beam passes through the optical element or is reflected by the same.
The aforementioned optical element made of glass, metal or ceramics is widely known. In recent years, a resin-made optical element has come to be employed in view of molding ease, greater freedom of designing, and reduced costs.
In the meantime, as a laser beam, the attention of the industry is focused on a short-wave light source having a wavelength of 500 nm or less, particularly in the vicinity of 400 nm in recent years because it provides high definition image recording, enhanced recording density, longer-service life and stable output. There is a need to use a resin-made optical element as a means for adjusting the optical path of the light emitted from this light source.
When the scanning optical device for adjusting the optical path of laser beam by means of a transparent type refraction optical element disclosed in the Patent Literature 1, namely, a transparent lens, is structured in such a way that the laser beam from the short-wave light source is used as the laser beam and a resin-made lens is used as the transparent lens for adjusting the optical path of laser beam, a short-wave laser beam will pass through the resin, and weatherability of the resin-made lens comes into question.
To solve the problem of weatherability, the present inventor considered use of a mirror for reflecting the laser beam, namely, a reflecting optical element as a means of adjusting the laser beam optical path, without using a transparent lens, namely, a transparent refraction type optical element that allows laser beam to pass through.
This is because of the following reason: A transparent optical element requires countermeasures to be provided to ensure weatherability, including the interior of the lens wherein light passes through. By contrast, the reflecting lens surface requires such countermeasures to be taken only for the reflecting optical element. This is a great advantage.
However, the following new problem arises in this case: When a reflecting optical element is used to adjust the optical path of the laser beam, the surface precision required for the profile of the reflecting optical surface is about four times that required for the transparent lens. This is because of the following reasons: In the case of the transparent lens, the entrance and exit surfaces of the lens are used to adjust the optical path of the laser beam and beam profile. By contrast, in the case of a reflecting optical element, the optical path of the laser beam and beam profile are adjusted by one reflecting surface.
In the optical element wherein the profile of the optical surface is required to ensure a high degree of surface precision, particularly in the optical element for adjusting the main scanning speed of the laser beam, there will be an increased impact on the deformation of the optical surface given by the warping or sink marks resulting from contraction during resin hardening, and on the warping produced in the direction of length, i.e., in the scanning direction, making it difficult to perform molding with such a high degree of surface precision maintained, even if the conventional injection molding procedure is utilized.
To solve this problem, the present inventors paid attention to the effect of the hollow injection molding, and considered application of hollow injection molding to the optical components. Tensile stress during resin contraction that may cause warping and sink marks of the molded products is released to the hollow portion by hollow molding according to hollow injection molding process. Then a sink marks appears on the surface of the hollow portion, thereby suppressing the warping and sink marks occurring on the surface of the hollow portion. This enhances surface precision, i.e., the mirror surface precision in the reflecting optical element.
The hollow injection molding technique is disclosed in Patent Literature 2. According to this technique, the hollow portion is designed wider than the mirror surface section of the reflecting optical element to achieve the effect of formation of a hollow portion over the entire mirror surface section. This technique will be advantageous in the sense the tensile stress during resin contraction is released to a certain extent by formation of a hollow portion over the entire mirror surface section.
However, this conventional technique has been found to be insufficient to maintain the surface precision required of the aforementioned reflecting optical element, particularly the reflecting optical element employed in the scanning optical device using the short-wave light source.